Inorganic glasses are highly transparent, heat-resistant and dimensions stability and, on account of these properties, they have been used from old days in a wide variety of industrial sectors as structures which divide the space while transmitting visible light without obstructing the visibility. In spite of such excellent properties, inorganic glasses have two grave shortcomings; first, they are heavy with the specific gravity amounting to 2.5 or more and, second, they have poor impact resistance and fracture easily. In recent years, as a result of the continuing drive toward downsizing such as reduction in weight and thickness of the products in all kinds of industries, there is an increasingly stronger demand from the users for improvements to remedy the aforementioned shortcomings.
Transparent thermoplastics and thermosets are being counted on as materials to meet such a demand from the industries. Transparent thermoplastics are exemplified by polymethyl methacrylate (PMMA) and polycarbonate (PC). Of these transparent thermoplastics, PMMA is otherwise called organic glass and it is highly transparent and drawing attention as a material which has overcome the two shortcomings of glasses. However, these transparent plastics are markedly inferior to inorganic glasses in heat resistance and coefficient of linear thermal expansion and face a problem of limited usage.
On the other hand, transparent thermosets are exemplified by epoxy resins, curable (meth)acrylic resins and silicone resins and they generally show higher heat resistance than the aforementioned thermoplastics. Of the transparent thermosets, epoxy resins show small curing shrinkage and excellent moldability, but have a shortcoming of low impact resistance and brittleness. Curable (meth)acrylic resins are well balanced in heat resistance, moldability and properties of molded articles, but have shortcomings of large changes in dimension by water absorption and in coefficient of linear expansion by heat.
Silicone resins are superior to other thermosets in heat resistance, weatherability and water resistance and provide materials with high potentialities of solving the aforementioned problems associated with plastics and serving as substitutes for inorganic glasses. In particular, polyorganosilsesquioxanes of a ladder structure are known to show heat resistance comparable to that of polyimides.
One example of such polyorganosilsesquioxanes is prepared as follows according to methods disclosed in JP40-15989, JP50-139900A and J. Polymer Sci., Part C, No. 1, pp. 83-97 (1963): phenyltrichlorosilane is hydrolyzed in an organic solvent to phenyltrihydroxysilane, the hydrolysis product is heated in a water-free solvent in the presence of an alkaline rearrangement and condensation catalyst to give cage type octaphenylsilsesquioxane and the cage type octaphenylsilsesquioxane is separated and heated again in the presence of an alkaline rearrangement and condensation catalyst to give a phenylsiloxane prepolymer of low intrinsic viscosity; or the prepolymer is further heated in the presence of an alkaline rearrangement and condensation catalyst to give a phenylsilsesquioxane polymer of high intrinsic viscosity.
Now, the siloxane linkage in silicone resins including the polyorganosilsesquioxanes prepared in the aforementioned manner is highly flexible and it is necessary to increase the crosslinking density in order to develop modulus required for structures. However, increasing the crosslinking density is undesirable as it markedly increases the curing shrinkage thereby rendering molded particles brittle. Furthermore, the curing shrinkage increases the residual stress and this makes it extremely difficult to obtain thick-walled molded articles. For this reason, silicone resins with a high crosslinking density are limited in use to coating applications and, at the present time, only silicone rubbers with a low crosslinking density are used in molding applications. A method for copolymerizing silicone resins with acrylic resins of good moldability is disclosed in the Journal of the Chemical Society of Japan, 571-580 (1998); according to this method, an acrylic polymer having alkoxysilyl side chains is used as a nonladder type silicone resin and it is copolymerized with an alkoxysilane to form a hybrid consisting of an acrylic polymer as organic ingredient and a polysiloxane as inorganic ingredient. However, silicone resins intrinsically show poor compatibility with acrylic resins and, in many cases, the optical properties, particularly light transmission, are damaged even when there is no problem with mechanical strength.
A molded article of a silanol-free silicone resin disclosed in JP10-251407A shows excellent heat resistance and optical properties. However, a silicone resin prepared from a cage type polyorganosilsesquioxane and a disiloxane containing a reactive functional group by equilibration reaction in the presence of an alkaline rearrangement and condensation catalyst has a small number of reactive functional groups, 1.1 on the average, in the molecule and is assumed to participate little in the three-dimensional crosslinked structure in the molded article. That is, increasing the proportion of silicone resin which contributes to characteristics such as heat resistance, weatherability and water resistance decreases the absolute number of reactive functional groups in the molded article and this in turn decreases the crosslinking density and hinders satisfactory construction of a three-dimensional crosslinked structure. As a result, the molded article shows deterioration in heat resistance and mechanical properties.